The present invention relates to an article formed of a grooved flexible graphite sheet which is fluid permeable in the transverse direction and has enhanced isotropy with respect to thermal and electrical conductivity. The article of the present invention is useful as a gas diffusing electrode in an electrochemical fuel cell.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. Graphites exhibit anisotropy because of their inherent structures and thus exhibit or possess many properties, like thermal and electrical conductivity and fluid diffusion, that are highly directional. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The natural graphites most suitable for manufacturing flexible graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Natural graphite flake which has been expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is at least about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least about 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g., roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, such as web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is at least about 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles, which generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
This very considerable difference in properties, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid like gases or liquids occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for improved gasket performance if the resistance to fluid flow parallel to the major surfaces of the graphite sheet (xe2x80x9caxe2x80x9d direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet (xe2x80x9ccxe2x80x9d direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces (xe2x80x9ccxe2x80x9d direction) of the flexible graphite sheet, and very substantially less in the direction parallel to and between the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction). In applications such as fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (xe2x80x9ccxe2x80x9d direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction).
The foregoing situations are accommodated by the present invention.
In accordance with the present invention, a graphite article is provided, comprising a compressed mass of expanded graphite particles in the form of a sheet having opposed first and second major surfaces with transverse fluid channels passing through the sheet between the first and second surfaces, with at least one of the surfaces having an open top groove interconnecting with a plurality of the transverse fluid channels. The transverse fluid channels passing through the sheet between the opposed first and second surfaces are advantageously formed by mechanically impacting a surface of the sheet to displace graphite within the sheet at a plurality of predetermined locations to provide the channels with openings at the first and second opposed surfaces. In a particular embodiment, the transverse channel openings at one of the parallel opposed surfaces are smaller than their respective openings at the other opposed surface whereby pressurized fluid in contact with the opposed surface having the smaller channel openings enters the respective channels at an initial velocity which is greater than the velocity of the fluid exiting the respective channels, i.e., the gas exit velocity is slowed. Likewise, pressurized fluid in contact with the opposed surface having the larger channel openings has higher gas exit velocity. The transversely channelled sheet is further mechanically impacted at one of its opposed surfaces, to displace graphite within the sheet and provide in the surface of the article a preferably continuous open top groove which interconnects with a plurality of the transverse fluid channels. The mechanical impacting can be suitably accomplished by molding, pressing and embossing. An open top groove can also be provided by engraving and etching techniques.
The article of the present invention is useful as a fluid permeable e.g. gas diffusing electrode having an integral gas diffusing element. In accordance with the present invention, a cover element for the grooved surface is provided in the form of roll-pressed and calendered anisotropic flexible graphite sheet which enhances heat transfer performance of the gas diffusing electrode in electrochemical fuel cells as hereinafter described.